It is well established that many medically significant biological processes are mediated by proteins participating in signal transduction pathways that involve G-proteins and/or second messengers, e.g., cAMP (Lefkowitz, Nature, 351:353-354 (1991)). Herein these proteins are referred to as proteins participating in pathways with G-proteins or PPG proteins. Some examples of these proteins include the GPC receptors, such as those for adrenergic agents and dopamine (Kobilka, B. K., et al., PNAS, 84:46-50 (1987); Kobilka, B. K., et al., Science, 238:650-656 (1987); Bunzow, J. R., et al., Nature, 336: 783-787 (1988)), G-proteins themselves, effector proteins, e.g., phospholipase C, adenyl cyclase, and phosphodiesterase, and actuator proteins, e.g., protein kinase A and protein kinase C (Simon, M. I., et al., Science, 252:802-8 (1991)).
For example, in one form of signal transduction, the effect of hormone binding is activation of an enzyme, adenylate cyclase, inside the cell. Enzyme activation by hormones is dependent on the presence of the nucleotide GTP, and GTP also influences hormone binding. A G-protein connects the hormone receptors to adenylate cyclase. G-protein was shown to exchange GTP for bound GDP when activated by hormone receptors. The GTP-carrying form then binds to an activated adenylate cyclase. Hydrolysis of GTP to GDP, catalyzed by the G-protein itself, returns the G-protein to its basal, inactive form. Thus, the G-protein serves a dual role, as an intermediate that relays the signal from receptor to effector, and as a clock that controls the duration of the signal.
The peptide endothelin is a peptide of 21 amino acid residues and performs in vivo effects via endothelin receptors. Endothelin (ET) is a peptide present in various tissues in animals and is known as a strong vasoconstrictor. ET is one peptide of a family of at least 4 mammalian peptides characterized by 2 disulphide bridges and 6 conserved amino acid residues at the C-terminus.
Members of the family are called endothelin-1 (ET-1), endothelin-2 (ET-2), and endothelin-3 (ET-3). A fourth peptide, vasointestinal contractor, is also sometimes described as the murine or rat form of ET-2. They differ mostly in the 29-membered ring system formed by the Cys-3-Cys-11 disulphide bond. Endothelins are produced by metabolism of a preproendothelin to a proendothelin, which is itself metabolized to the mature endothelin. The cleavage of proendothelin is thought to be due to the activity of a specific enzyme. ETs are distributed in a wide variety of vascular and non-vascular tissues (PNAS, USA, 86:2863-2867 (1989)).
It has previously been shown in vivo that ET-1 and ET-2 are much stronger vasoconstrictors than ET-3, whereas the three ET isopeptides are roughly equipotent in producing the transient vasodilation. The analysis of nucleic acid sequences of ETs has revealed that various kinds of ET isopeptides exist. These ET isopeptides are also different in their properties. Therefore, it appears that various sub-types of ET-receptors exist. The existence of various sub-types of ET-receptors has been proven by the radioactive ligand binding studies of Watanabe, H., et al., Biochem-Biophys, Res. Commun., 161: 1252-1259 (1989), and Martin, E. R., et al., J. Biol. Chem., 265:14044-14049 (1990). These studies indicate the existence of at least two kinds of ET-receptors. One of them has a higher affinity for ET-1 and ET-2 than for ET-3 and the other has an affinity for ET-1, ET-2 and ET-3 with no cell activity. The ETA receptors have a lower affinity for ET-3 and the ETB receptors are non-selective.
The receptors are homologous to other heptahelical receptors of the rhodopsin superfamily, having 7 hydrophobic regions predicted to form transmembrane helices.
The placenta has a very high expression of both receptors, as does the lung. In general the non-selective ETB receptor seems to be more widely expressed (e.g., in liver, kidney and uterus) and is probably the more prominent receptor in the CNS, a result that agrees with binding and functional studies. The heart is the only tissue about which there is a consensus that an ETA-type receptor predominates. The ETA receptors are associated with blood vessels and ETB receptors with glial, epithelial and ependymal cells, but few, if any, are associated with neurons. In the kidney, ETA receptors are located on blood vessel smooth-muscle cells, and ETB receptor expression occurs on a glomerular endothelium, vasa recti and the thin segments of Henle""s loops.
Endothelins elicit biological responses by various signal transduction mechanisms, including the G-protein-coupled activation of phospholipase C and the activation of voltage-dependent Ca2+ channels (Kasuya, Y., et al., Biochem. Biophys. Res. Commun., 61:1049-1055 (1989)). Thus, different sub-types of the endothelin receptor may use different signal-transduction mechanisms. Endothelin receptors have a relatively long N terminus preceding transmembrane segment I, and this portion may be involved in binding a relatively large endothelin peptide.
Applicants have discovered a G-protein coupled receptor which has hydropathicity and amino acid homology which shows the existence of the 7 hydrophobic segments and a significant sequence similarity with other G-protein-coupled receptors. The 7 membrane-spanning domains and extra-cellular N-terminus and cytoplasmic C-terminus have also been identified.
The G-protein coupled receptor of the present invention has been putatively identified as an endothelin-bombesin receptor as a result of its homology to the known endothelin receptors ETA and ETB and as a result of its ability to bind endothelin and bombesin.
In accordance with one aspect of the present invention, there is provided a novel putative mature polypeptide which is a G-protein coupled receptor, as well as fragments, analogs and derivatives thereof. The polypeptide of the present invention is of human origin.
In accordance with another aspect of the present invention, there are provided polynucleotides (DNA or RNA) which encode such polypeptides.
In accordance with yet a further aspect of the present invention, there is provided a process for producing such polypeptides by recombinant techniques.
In accordance with yet a further aspect of the present invention, there is provided a process for utilizing such polypeptides, or polynucleotides encoding such polypeptides for therapeutic purposes, to measure the concentration of endothelin in vivo, or in soluble form as an antagonist.
In accordance with yet a further aspect of the present invention, there are provided antibodies against such polypeptides.
In accordance with yet another aspect of the present invention, there are provided antagonists to such polypeptides, which may be used to inhibit the action of such polypeptides.
In accordance with still another embodiment, there is provided a process for using the receptor to screen for receptor antagonists and/or receptor agonists and/or receptor ligands.
These and other aspects of the present invention should be apparent to those skilled in the art from the teachings herein.
The following drawings are illustrative of embodiments of the invention and are not meant to limit the scope of the invention as encompassed by the claims.